1. Field of the Invention
The present invention relates to a contact charger for use in an electrophotographic image forming apparatus such as a copying machine or a printer, and also relates to an image forming apparatus provided with such a contact charger.
2. Description of Related Art
<Corona Charger>
Conventional image forming apparatuses such as an electrophotographic device have employed corona chargers, which utilize corona discharging for charging a charging target (i.e., an object to be charged) such as a photosensitive member for electrophotography.
The corona charger is arranged in a noncontact manner with respect to the charging target, and is configured such that a high voltage is applied, e.g., to a wire electrode or a needle electrode for causing corona discharging, and thereby a part of discharge current thus caused flows through the charging target to place a predetermined potential on the charging target.
However, the corona charger utilizing the corona discharging generates a large amount of ozone, which causes a problem in environment due to ozone smell. Also, a large amount of ozone deteriorates the charging target, and/or a discharging product due to corona discharging (i.e., material produced by corona discharging) may adhere to a surface of the charging target. Thereby, quality of images is impaired, and/or the charging target has to be shaved for recovery from deterioration due to the adhered discharging product so that the life thereof is reduced. Further, a power source of a high voltage and therefore an expensive power source are required.
<Contact Charger (Charging by Discharging)>
In recent years, therefore, many contact chargers have been proposed for use instead of the corona chargers. For example, a roller charger, a fur-brush charger, a blade charger and others have been proposed. These chargers are configured to charge the charging target by utilizing a discharging phenomenon, which occurs between the charging target and the charging member. The charging member is arranged in direct contact with the charging target, and a voltage is applied to the charging member to place a predetermined potential on the charging target.
The roller charger includes, e.g., an elastic roller having an electrically conductive elastic layer. The elastic roller is in contact with the charging target to form a nip, and a voltage is applied to the elastic roller to charge the charging target. In many structures, the elastic roller is driven to rotate by the charging target.
The fur-brush charger is formed of, e.g., a fur-brush roller having electrically conductive brush fibers. The fur-brush roller is in contact with the charging target to form a nip, and a voltage is applied to the fur brush to charge the charging target. Since the fibers used therein are extremely thin, a strong electric field is locally produced between the fur brush and the charging target, and excessive discharging not following Paschen's law occurs in the strong electric field so that irregular charging occurs. Since the contact between the charging target and the brush fibers consists of a gathering or combination of line-contact and/or point-contact, it is difficult to ensure a sufficiently large contact area between the charging target and the fur brush so that it is impossible to prevent insufficient charging due to insufficient contact.
These contact chargers can charge the target with power sources of lower voltages than those of the corona charger. In these contact chargers, however, a voltage prepared by adding a threshold voltage for following Paschen's law to an intended charging potential must be applied to the charging member. Further, the amount of produced ozone can be smaller than that of the corona charger, but disadvantages due to the discharging product are unavoidable because the charging operation utilizes the discharging phenomenon.
<Contact Charger (Injection Charging)>
For overcoming the above problems, such a contact charger has been proposed that injects electric charges directly into a charging target without utilizing the discharging phenomenon. For example, a magnetic brush charger, a roller charger, a fur-brush charger and others have been proposed as the contact chargers utilizing injection charging.
These chargers are configured to charge the charging target to bear an potential substantially equal to the voltage applied to the charging member, and therefore can utilize a charging voltage lower than that of the foregoing contact charger utilizing the discharging phenomenon. Further, the discharging does not occur so that the discharging product is not generated, and disadvantages due to the discharging product do not occur.
The magnetic brush charger is formed of, e.g., a nonmagnetic sleeve covering a magnetic roller, and electrically conductive and magnetic particles retained on the sleeve. A spike (magnetic brush) formed of the conductive and magnetic particles is in contact with the charging target to form a nip, and a voltage is applied to the magnetic brush to charge the charging target by charge injection. This type of charger requires a complicated structure, and therefore is expensive. Further, it suffers from dropping of conductive and magnetic particles as well as image noises due to adhesion of the conductive and magnetic particles onto the charging target such as a photosensitive member.
The roller charger is formed of, e.g., a conductive and elastic roller. The conductive and elastic roller is brought into contact with the charging target to form a nip, and a voltage is applied to the elastic roller to effect injection charging on the charging target. For effecting the injection charging on the charging target, a sufficient contact area is required between the roller surface and the charging target. However, such a sufficient contact area cannot be achieved if the elastic roller is merely driven to rotate by the charging target. For obtaining the sufficient contact area, a difference may be provided between peripheral speeds of the elastic roller and the charging target so that the elastic roller may slide on the charging target.
However, since the elastic roller is in face-contact with the charging target, a large frictional force occurs. Thereby, the surfaces of the charging member and the charging target may be shaved to generate image noises, and the durability thereof may be reduced.
For reducing the frictional force, Japanese Laid-Open Patent Publication No. 10-307458 has disclosed a roller charger, in which conductive particles are disposed in a contact nip between the roller charger and the charging target. Even in this structure, a frictional force is larger than that in the chargers, which utilize line-contact and/or point-contact of a fur-brush or a magnetic brush, and therefore, the charging member and the charging target are shaved so that image noises occur, and low durability is unavoidable.
For example, U.S. Pat. Nos. 6,081,681 and 6,289,190 have disclosed fur-brush chargers, in which a fur brush carrying conductive particles is in contact with the charging target to form a nip, and a voltage is applied to the fur brush to perform injection charging on the charging target. Since the fur brush is in line-contact and/or a point-contact with the charging target, a frictional force between them is small, and wearing of the charging member and the charging target is considerably suppressed. Further, the discharging phenomenon is not utilized so that irregular charging due to excessive discharging can be prevented. Since the conductive particles are present between the charging target and the fur brush, insufficient contact between the fur brush and the charging target can be suppressed, as compared with the foregoing fur brush charging utilizing the discharging phenomenon.
However, according to the conventional fur brush injection charger described above, it is impossible to lower sufficiently a contact resistance (electric resistance) between the fur brush and the charging target so that uniform charging cannot be performed in some cases. In an example disclosed in foregoing U.S. Pat. No. 6,289,190, a fur brush of 14 mm in outer diameter is in contact with a photosensitive member of 30 mm in diameter, and a nip of 3 mm in width is formed for charging the photosensitive member.
However, this U.S. patent has not taught an appropriate range of the push-in amount of the fur brush with respect to the photosensitive member, which is important for reducing the contact resistance between the fur brush and the photosensitive member. The injection charging is primarily performed in the contact area between the charging member and the charging target. However, even if the contact area is sufficiently large, uniform charging without irregularity cannot be performed when the contact resistance is large. It is necessary to ensure an appropriate push-in amount in addition to an appropriate contact area.